Implantable valve accumulator pump systems for the delivery of mediation or other fluids to a patient are well known and described in U.S. Pat. Nos. 4,838,887 and 5,049,141, the disclosures of which are hereby incorporated by reference.
U.S. Pat. Nos. '887 and '141 disclose an implantable valve accumulator pump system for the delivery of infusate, such as medication. The implantable pump portion is comprised of four essential assemblies, as shown in FIG. 1. The first major assembly is a rechargeable, constant pressure drug reservoir 10 in series with a bacteria/air filter 24. In one embodiment, the reservoir 10 comprises a sealed housing 14 containing a bellows 16. The bellows 16 separates the housing 14 into two parts. Chamber 18 is used to hold the drug or other medicinal fluid. Second zone 20 is normally filled with a two-phase fluid, such as Freon®, that has a significant vapor pressure at body temperature. Thus, as the fluid within the second zone 20 vaporizes, it compresses the bellows 16, thereby pressurizing the drug in the chamber 18. The drug can be refilled via septum 12.
The two-phase fluid helps maintain the chamber 18 under a constant pressure. When the chamber is refilled, the two-phase fluid is pressurized thereby condensing a portion of the vapor and converting it to liquid. As the chamber 18 is emptied, this liquid vaporizes, thus maintaining the pressure on the bellows 16.
Since the infusate in chamber 18 is under positive pressure, it is urged out of the chamber, through a bacterial filter 24 and toward the metering assembly.
The second major assembly is an electronically controlled metering assembly comprising two normally closed solenoid valves 26, 28, which are positioned on the inlet and outlet sides of a fixed volume accumulator 30. The valves are controlled electronically via a module 32, which can be programmed utilizing an external programmer 34. The metering assembly is designed such that the inlet valve 26 and the outlet valve 28 are never simultaneously open.
The third major assembly is an outlet catheter 36 for medication infusion in a localized area. The delivery of fluid occurs at an infusion site that is below the accumulator pressure, thereby forcing discharge through the catheter 36.
The fourth assembly of this system is the external programmer 34 used to communicate and program the desired medication regimen. This programmer is preferably a handheld unit with a touch screen. It provides a data transfer link to the implanted electronics 32 and is able to exchange information with the electronics 32, including but not limited to battery status, diagnostic information, calibration information, etc.
Returning to the metering assembly, FIG. 2 illustrates the normal sequence used to fill and dispense infusate. The valves in the medication metering assembly alternately open and close to admit infusate from the reservoir 18 into the accumulator 30, via conduit 22, and to dispense a precise volume spike to an outlet catheter 36. During the first step, both valves are closed and the accumulator is empty. In this step, no fluid is moved. During the second step, the inlet valve 26 opens while the outlet valve 28 remains closed. Since the incoming fluid is at a higher pressure than the accumulator 30, fluid fills the accumulator. The accumulator preferably has a fixed volume such that exact amounts of fluid can be dispensed. Once the accumulator 30 is filled, no fluid movement occurs. During the third step, the inlet valve 26 closes, thereby separating the reservoir from the accumulator. At this step, the accumulator 30 is filled. Finally, during the fourth step, the outlet valve 28 opens. Since the accumulator 30 is at a higher pressure than the outlet canella, the fluid exits the accumulator through outlet valve 28.
FIG. 3 illustrates the components used in the metering assembly of the prior art. Valves 26 and 28 are implemented as miniature solenoid valves. The valves are preferably disposed in a side-by-side arrangement having two solenoid assemblies 74, each receiving power via a corresponding electrical lead 76. The valves are operably powered to drive a working plunger 78 biased by means of spring 80. The working plunger and return spring assembly are isolated from the solenoids 74 by means of an isolation diaphragm 82. As is customary, the solenoid is actuated by a magnetic field that drives the working plunger 78. Once charged, the solenoid overcomes the force of the bias spring 80, and pulls the plunger 78 off the valve seat 84, allowing fluid flow.
The flow path of the infusate or medicinal fluid is illustrated by the arrows in FIG. 3. As described above, with valve 26 in the open position, fluid communication is established between the accumulator 30 and the inlet conduit 54. The infusate is thereby delivered upward through the valve seat 84 (shown closed in FIG. 3), into the accumulator flow passage 86. The area between the valve seats comprises the accumulator storage space. When valve 26 is closed, the accumulator 30 is isolated from the reservoir 18.
When valve 28 is opened, fluid communication is established between the accumulator and the outlet conduit 55. The infusate is thereby delivered downward from the accumulator storage space, through the valve seat 84 (shown closed in FIG. 3), and into the outlet conduit 55. Furthermore, the system is preferably designed such that valves 26 and 28 cannot be opened at the same time in order to prevent the metering function of the accumulator 30 from being bypassed.
This system is highly effective in most situations. However, when a patient with such a device enters an MRI (magnetic resonant imaging) environment, the presence of large magnetic fields above a certain threshold may affect the operation of the pump's valves such that the metering function of the accumulator is bypassed. Consequently, patients implanted with these devices must be instructed to have the pump reservoir 18 emptied before undergoing an MRI procedure. This prohibition and warning is commonplace for patients implanted with such medical devices.
It would be beneficial for such an implantable drug delivery system to remain operational even in the presence of strong magnetic fields, such as those encountered during MRI procedures.